Cardiac resynchronization cardiac pacing devices operate by either delivering pacing stimulus to both ventricles or to one ventricle with the desired result of a more or less simultaneous mechanical contraction and ejection of blood from the ventricles. However, due to a number of factors for a variety of patients such cardiac pacing systems may not always effectively delivery CRT. For example, varying capture thresholds, pacing lead and/or electrode migration or dislodgement, time required for appropriate signal processing, confounding conduction delays or conduction blockages, diverse electrode placement locations, and the like.
In either form of CRT delivery, whether fusion-based or the more traditional bi-ventricular stimulation, confirming that pacing stimulus captures each paced ventricle is a very important clinical issue so that the desired benefits of the CRT are in fact delivered to a patient.
Assuming that the reader is familiar with bi-ventricular pacing, the following should provide additional insight into the importance of capture detection in a fusion-based bi-ventricular pacing engine. One premise underlying fusion-based pacing is the notion that a fusion-based evoked left ventricular (LV) depolarization enhances stroke volume in hearts where the right ventricle (RV) depolarizes first. This is commonly due to intact atrio-ventricular (AV) conduction to the RV of a preceding intrinsic or evoked atrial depolarization wave front, and wherein the AV conducted depolarization of the LV is unduly delayed. The fusion depolarization of the LV is attained by timing the delivery of the LV pace (LVp) pulse to follow the intrinsic depolarization of the RV but to precede the intrinsic depolarization of the LV. Specifically, an RV pace (RVp) pulse is not delivered due to the inhibition of the RVp event upon the sensing of RV depolarization (RVs), allowing natural propagation of the wave front and depolarization of the intraventricular septum, while an LVp pulse is delivered in fusion with the RV depolarization. For supporting mode switches to alternate pacing modalities, fusion-based CRT delivery engines typically include at least one electrode in each ventricle which allows such engines to be used in conjunction with the present invention, as will be apparent upon review of the following written description and drawings of the invention.
Left ventricular capture in particular is a clinical issue with present-generation (and foreseeable) CRT systems, due to acknowledged difficulty of maintaining stable lead situation in the cardiac venous anatomy. Since CRT delivery becomes ineffective (possibly even deleterious) if LV capture is lost, diagnosis of dislodgment and maintenance of capture are high priorities.
Cardiac Resynchronization Therapy (CRT) devices have been shown to improve quality of life (QOL), exercise capacity and New York Heart Association (NYHA) heart failure class. The NYHA rating varies from Class I to Class IV, as follows: Class I: patients with no limitation of activities; they suffer no symptoms from ordinary activities. Class II: patients with slight, mild limitation of activity; they are comfortable with rest or with mild exertion. Class III: patients with marked limitation of activity; they are comfortable only at rest. Class IV: patients who should be at complete rest, confined to bed or chair; any physical activity brings on discomfort and symptoms occur at rest.
Currently approved CRT devices incorporate bi-ventricular pacing technology with simultaneous pacing in the right ventricle (RV) and the left ventricle (LV). Since the devices are implanted essentially only to provide continuous bi-ventricular pacing therapy, it is imperative that each pacing pulse stimulus delivered to the two LV and RV provide an evoked response (i.e., each stimulus delivered to a ventricle “captures” the ventricle). Thus, if electrodes disposed in electrical communication with a ventricle rapidly sense depolarization wavefronts a control sequence for the pacing engine will inhibit ventricular pacing. For example, such a situation occurs during rapidly conducted atrial fibrillation (AF). When bi-ventricular pacing is inhibited the patient's symptoms of heart failure return, and can sometimes even worsen as compared to their pre-implant status. Similarly, if one of the pacing sites loses capture (e.g., the LV) the subsequent RV-only pacing will prevent the patient from receiving the intended benefit of CRT delivery. To that end the inventors have addressed a need in the art regarding capture verification in heart failure devices, such as bi-ventricular CRT devices that indicates when capture is occurring in both the LV and the RV.
Presently, the only somewhat similar diagnostic available in CRT devices is percent-ventricular pacing (% Vpacing), which indicates the percentage of time bi-ventricular pacing therapy is being delivered; however, a limitation of the % Vpacing metric is that bi-ventricular pacing may be “occurring” close to 100% of the time but the LV chamber may not be captured at all. Currently, cardiac device specialists assess LV capture acutely during office visits by looking at the morphology of an electrogram (EGM) or by temporarily setting pacing to RV-only and LV-only pacing. Current state of the art pacemakers (e.g., the Kappa® brand family of pacemakers provided by Medtronic, Inc.) incorporate ventricular capture management algorithms. However, such algorithms require specific circuitry and sensing capabilities to be able to perform this function that are not currently available in the CRT products. Also, the feasibility of this technology for LV capture management has yet to be established. The present invention advantageously contributes to both capture verification and management.
Previously others addressed issues related to capture management; for example, Ventricular Capture Management (VCM) has been successfully implemented in the Kappa® 700 dual-chamber pacemaker sold by Medtronic, Inc. by measuring evoked responses on the bipolar pair of electrodes in the right ventricle (RV). In this device the pacing output energy is monitored and automatically adjusted as required by the patient. This pacing threshold search (PTS) measures the rheobase and chronaxie of the current pacing threshold. The following can be used to determine rheobase and chronaxie: 1—determine the rheobase, which is the minimum Stimulus Strength that will produce a response (his is the voltage to which the Strength-Duration curve asymptotes). Step 2—calculate 2×rheobase and step 3—determine chronaxie, which is the Stimulus Duration that yields a response when the Stimulus Strength is set to exactly 2×rheobase.
Then, a pulse width and amplitude safety margin is calculated and the output of the device is set to that new value. The PTS is conducted on a programmable periodic basis, commonly set up to measure the thresholds once a day (typically at night).
Currently in the bi-ventricular pacing CRT devices like the InSync® family of implantable pulse generators, including ICDs), no capture verification or threshold management scheme exists. Instead, pacing thresholds are manually measured at the right ventricular and the left ventricular pacing sites. The site with the highest pacing threshold requirement dictates the programmed output of the device to assure proper capture at both ventricular sites for devices with a single ventricular pacing stimulus energy output.
A need therefore exists in the art to effectively chronically deliver ventricular pacing therapies (including CRT) to patients who might not otherwise receive the full benefit of such therapies.